Implanted medical devices such as heart pacemakers are often programmed to use telemetric signals that are generated by a remote programming unit. Verification of the programming of implanted device is provided by the transmission of signals from the device to a receiving section of the programming unit. The programming signals are digital signals which are coded in some manner to signify logic 1 and 0 signals. In the disclosed embodiment, the encoding employs pulse interval modulation, wherein the intervals between bursts of high frequency pulses are long or short depending on the logic level of the data bit being transmitted.
Signals from implanted devices in a hospital environment are often subject to a relatively high level of noise, or interference. As long as the signal level is greater than the noise level, however, the gain of the receiver may be adjusted so that the noise level will be less than a threshold level. The information content of the coded signal is then employed to adjust the gain of the receiver. When errors occur in the message being sent from the implanted device, the number of errors in the repeated signal are counted over a period of time. If more than a predetermined number of errors occur in that time, the gain will be adjusted by a predetermined amount, and the signal will be continually monitored to determine if the adjusted gain level has eliminated the errors. The adjustment continues until all of the bits of the encoded information are received without error, and then further processing of received data from the implanted device is allowed to proceed.